Histone acetylation is a reversible process whereby histone and non-histone protein acetyl-transferases transfer the acetyl moiety from acetyl co-enzyme A to lysines and histone deacetylases (HDACs) remove the acetyl groups establishing the positive charge in the proteins. There are eighteen HDACs in humans of which eleven are zinc-dependent designated class I (HDACs 1, 2, 3, 8), Class IIa (HDACs 4, 5, 7, 9), Class IIb (HDACs 6, 10), and Class IV (HDAC 11) (Marks & Xu J Cell Biochemistry E-pubmed, 2009) (Table 1). Among the zinc-dependent HDACs, class I HDACs are primarily localized in the nucleus while class II HDACs are primarily cytoplasmic in location but shuttle between the nucleus and the cytoplasm (see reviews: Bolden et al. Nat Rev Drug Discovery 5:769-84, 2006; Glozak & Seto Oncogene 26:5420-32, 2007; Dokmanovic et al. Mol Cancer Research 5:981-989, 2007; Marks & Xu J Cell Biochemisty E-pubmed, 2009). The accumulating evidence indicates that these HDAC enzymes are not redundant in their biological activity.
In addition to histones, HDACs have many non-histone protein substrates that have a role in regulation of gene expression, cell proliferation, cell migration, cell death and angiogenesis. HDAC inhibitors cause the accumulation of acetylated forms of protein substrates and alter the structure and function of these proteins. HDAC inhibitors can induce different phenotypes in various transformed cells including growth arrest, apoptosis, reactive oxygen species-facilitated cell death and mitotic cell death. Normal cells are relatively resistant to HDAC inhibitor-induced cell death.
Among the eleven zinc-dependent HDACs, HDAC 6 is unique. HDAC 6 has two identical catalytic sites, a ubiquitin-binding site toward its C-terminal end and is primarily cytoplasmic in location. HDAC 6 is a known specific deacetylase of several proteins including α-tubulin, cortactin, peroxiredoxins, chaperone proteins, HSP90, β-Catenin, hypoxia inducible factor-1α (HIF-1α) and other proteins, but not histones in viva (see above cited reviews, and Blackwell et al., Life Science 82:1050-1058, 2008; Shnakar & Sirvastava Adv Exp Med Biol 615:261-298, 2008). A previously unrecognized substrate of HDAC 6 was recently discovered, namely, peroxiredoxins, which are proteins critical in protecting cells from the oxidative effects of H2O2 (Parmigiani et al. PNAS 105:9633-9638, 2008).
Suberoylanilide hydroxamic acid (vorinostat) is an inhibitor of class I HDAC 1, 2, 3, and 8, class IIb HDACs and 10, and class IV HDAC 11 (Marks & Breslow Nat Biotechnol 25:84-90, 2007). Tubacin (Haggerty et al. PNAS 100:4389-4394, 2003) (Table 2) and compound 7 (in Kozikowski et al. J. Med Chem 51:4370-4373, 2008) are selective HDAC 6 inhibitors as indicated by assays with purified recombinant zinc-dependent HDACs including HDAC 6 and HDAC 1.
Compound 7 is designated herein as BAHA (Table 2). As indicated above, HDAC 6 selectively deacetylates a number of proteins that have a role in regulating cell proliferation, cell migration, cell death and angiogenesis. Inhibition of HDAC 6 causes accumulation of acetylated forms of these proteins, altering their structure and function that can cause inhibition of cell proliferation, cell migration and metastasis and angiogenesis.